Monitoring toxic gases is a great concern in relation to environmental pollution, occupational health, and industrial emission control. Known methods and apparatuses have been developed to detect the presence of gas. For example, gas chromatography, ion chromatography, electrolytic conductivity detection, and conductometric measurement are typically used to detect gas. However, these manners for detecting gas have generally been expensive, cumbersome, shown to have poor sensitivity and slow response times. They also typically cannot readily be used for on-line measurements. Other manners for monitoring include capacitance sensors and surface acoustic wave sensors. However, the sensitivities, or detection capabilities, of these devices generally fall in the range of low-ppm to high-ppb.
Electrochemical sensors were provided to overcome these limitations. Electrochemical sensors typically operate at room temperature, provide a signal which varies with concentrations of analyte species, have short response time, and exhibit acceptable sensitivity, stability, and reproducibility. In addition, electrochemical sensors are compact and can be used for continuous monitoring. Known electrochemical sensors include both liquid and solid electrolytic layers.
Other known electrochemical gas sensors typically include metal layers or gas diffusion electrodes in contact with an electrolytic film of, for example, Nafion or Teflon. The cornerstone of these sensors generally has been on optimizing the metal/gas/ionic medium interface in order to achieve higher sensitivity. However, the assembly processes for these sensors are manually intensive and are not suited for automated mass production.
Recently, planar thin film sensors have been developed by constructing three planar electrodes on an insulating substrate and covering them with a thin polymer electrolyte, such as Nafion. J. A. Cox and K. S. Alber, Amperometric Gas Phase Sensor for the Determination of Ammonia in a Solid State Cell Prepared by a Sol-Gel Process, 143, No. 7 J. Electrochem. Soc. L126-L128 (1996) developed a solid state cell in which microelectrode arrays were coated with a film of vanadium oxide xerogel for detection of ammonia. However, this film needs to be soaked in an electrolyte solution in order to provide ionic conductivity. These methodologies, in which a planar substrate with metal electrodes is covered with a thin film of solid state electrolytic material, are suitable for automated mass production, but they have lower sensitivity since gas needs to diffuse through a relatively thick film of electrolyte.
U.S. Pat. No. 5,866,204 to Robbie et al. (“'204 patent”) discloses a method of making a vapor deposited thin film by introducing vapor at varying angles onto a substrate. The '204 patent claims a film forming method that allows for the growth of complex microstructures with predetermined patterns of growth and porosity. However, the '204 patent does not disclose use of the vapor deposited thin film for detecting gas or in conjunction with an electrochemical sensor. Furthermore, no reference discloses depositing metal, for use as an electrode, on top of an electrolytic film.
What is desired, therefore, is to provide an electrochemical sensor suitable for mass production and having improved sensitivity. What is further desired is to provide an electrochemical sensor having detection capabilities desirably in the range of sub-ppb to low ppb levels. What is yet further desired is to provide an electrochemical sensor having improved response time.